1. Field of the Invention
The present invention relates to a radiographic imaging apparatus and a radiographic imaging method for carrying out calibration of a radiation detector when a radiographic image of a subject is taken using the radiation detector, as well as a program for causing a computer to function as the radiographic imaging apparatus.
2. Description of the Related Art
Conventionally, various types of radiation detectors (so-called “flat panel detectors”, which are hereinafter referred to as “FPDs”), which record a radiographic image of a subject formed by radiation transmitted through the subject, have been proposed and reduced into practice in the medical field, etc. An example of such a FPD is a FPD using a semiconductor, such as amorphous selenium, which generates an electric charge when exposed to radiation. As this type of FPD, those of so-called optical reading system and of TFT reading system have been proposed. Further, various types of cassettes, which include, in a case thereof, a FPD and an image memory serving as storage means for storing radiographic image data outputted from the FPD, have been proposed. Furthermore, among this type of cassettes, those provided with a function to send radiographic image data detected by the FPD to a processor via wireless communication have been proposed, so that the processor applies signal processing, such as image processing, to the radiographic image data.
When a radiographic image is taken using the FPD, it is necessary to carry out calibration before actual imaging to correct for variation in sensitivity, etc. To this end, various techniques for carrying out calibration in an imaging apparatus have been proposed (see U.S. Pat. No. 7,519,156, Japanese Unexamined Patent Publication Nos. 2005-111054 and 2002-336225, which are hereinafter referred to as Patent Documents 1 to 3, respectively). In these techniques, calibration is carried out when a power supply to the FPD is turned on or when a predetermined time period has elapsed after the power supply is turned on, for example, and calibration data obtained during the calibration is stored in the imaging apparatus with being associated with the FPD or a cassette containing the FPD. Then, during imaging, radiographic image data obtained by the imaging is corrected using the calibration data. Further, a technique to detect a newly connected cassette and carry out calibration has been proposed (see Japanese Unexamined Patent Publication No. 2005-204857, which is hereinafter referred to as Patent Document 4).
Since the FPDs deteriorate as an accumulated time for which the power is supplied thereto increases, it is necessary to update the calibration data on a regular basis. In the techniques disclosed in Patent Documents 1 to 4, however, timing of updating the calibration data is not mentioned. Therefore, the calibration may be carried out even at timing when it is unnecessary. Further, in the case of a cassette capable of wireless communication, the cassette contains a battery for driving the radiation detector. In this case, the power supply is frequently turned on or off in order to prevent wasteful power consumption by the battery. If the timing of calibration is set such that the calibration is carried out each time the power supply is turned on, it is impossible to carry out imaging immediately after the power supply is turned on, and this impairs efficiency of the imaging.